Transcript Slide 1

PH0101 Unit 2 Lecture 6
Klystron Oscillator
Reflex Klystron
Traveling Wave Tube
Biological effect of microwaves
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Klystron Oscillator
A klystron is a vacuum tube that can be
used either as a generator or as an
amplifier of power, at microwave
frequencies.
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Two cavity Klystron Amplifier
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Applications
 As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter transmitters
3. satellite communication ground station
4. radar transmitters
 As power oscillator (5 – 50 GHz), if used
as a klystron oscillator
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Reflex Klystrons
The reflex klystron has been the most
used source of microwave power in
laboratory applications.
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Construction
 A reflex klystron consists of an electron gun, a cavity with a
pair of grids and a repeller plate as shown in the above
diagram.
 In this klystron, a single pair of grids does the functions of
both the buncher and the catcher grids.
 The main difference between two cavity reflex klystron
amplifier and reflex klystron is that the output cavity is
omitted in reflex klystron and the repeller or reflector
electrode, placed a very short distance from the single
cavity, replaces the collector electrode.
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Working
 The cathode emits electrons which are accelerated
forward by an accelerating grid with a positive voltage on it
and focused into a narrow beam.
 The electrons pass through the cavity and undergo
velocity modulation, which produces electron bunching
and the beam is repelled back by a repeller plate kept at a
negative potential with respect to the cathode.
 On return, the electron beam once again enters the same
grids which act as a buncher, therby the same pair of grids
acts simultaneously as a buncher for the forward moving
electron and as a catcher for the returning beam.
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Reflex Klystron oscillator
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Working
 The feedback necessary for electrical oscillations is
developed by reflecting the electron beam, the velocity
modulated electron beam does not actually reach the
repeller plate, but is repelled back by the negative voltage.
 The point at which the electron beam is turned back can
be varied by adjusting the repeller voltage.
 Thus the repeller voltage is so adjusted that complete
bunching of the electrons takes place at the catcher grids,
the distance between the repeller and the cavity is chosen
such that the repeller electron bunches will reach the
cavity at proper time to be in synchronization.
 Due to this, they deliver energy to the cavity, the result is
the oscillation at the cavity producing RF frequency.
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Performance Characteristics
1.
2.
3.
4.
5.
Frequency: 4 – 200 GHz
Power: 1 mW – 2.5 W
Theoretical efficiency : 22.78 %
Practical efficiency : 10 % - 20 %
Tuning range : 5 GHz at 2 W – 30 GHz
at 10 mW
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Applications
 The reflex klystrons are used in
1. Radar receivers
2. Local oscillator in microwave receivers
3. Signal source in microwave generator of
variable frequency
4. Portable microwave links
5. Pump oscillator in parametric amplifier
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Traveling Wave Tube
Traveling Wave Tube (TWT) is the most versatile
microwave RF power amplifiers.
The main virtue of the TWT is its extremely wide
band width of operation.
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Basic structure of a
Traveling Wave Tube (TWT)
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Basic structure
 The basic structure of a TWT consists of a cathode and
filament heater plus an anode that is biased positively to
accelerate the electron beam forward and to focus it into a
narrow beam.
 The electrons are attracted by a positive plate called the
collector, which has given a high dc voltage.
 The length of the tube is usually many wavelengths at the
operating frequency.
 Surrounding the tube are either permanent magnets or
electromagnets that keep the electrons tightly focused into
a narrow beam.
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Features
 The unique feature of the TWT is a helix or coil that
surrounds the length of the tube and the electron beam
passes through the centre or axis of the helix.
 The microwave signal to be amplified is applied to the
end of the helix near the cathode and the output is
taken from the end of the helix near the collector.
 The purpose of the helix is to provide path for RF
signal.
 The propagation of the RF signal along the helix is
made approximately equal to the velocity of the electron
beam from the cathode to the collector
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Functioning
 The passage of the microwave signal down the
helix produces electric and magnetic fields that
will interact with the electron beam.
 The electromagnetic field produced by the helix
causes the electrons to be speeded up and
slowed down, this produces velocity modulation
of the beam which produces density modulation.
 Density modulation causes bunches of electrons
to group together one wavelength apart and.
these bunch of electrons travel down the length
of the tube toward the collector.
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Functioning
 The electron bunches induce voltages into the
helix which reinforce the voltage already present
there. Due to that the strength of the
electromagnetic field on the helix increases as the
wave travels down the tube towards the collector.
 At the end of the helix, the signal is considerably
amplified. Coaxial cable or waveguide structures
are used to extract the energy from the helix.
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Advantages
1. TWT has extremely wide bandwidth. Hence, it
can be made to amplify signals from UHF to
hundreds of gigahertz.
2. Most of the TWT’s have a frequency range of
approximately 2:1 in the desired segment of the
microwave region to be amplified.
3. The TWT’s can be used in both continuous and
pulsed modes of operation with power levels up
to several thousands watts.
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Performance characteristics
1.
2.
3.
Frequency of operation : 0.5 GHz – 95 GHz
Power outputs:
5 mW (10 – 40 GHz – low power TWT)
250 kW (CW) at 3 GHz (high power TWT)
10 MW (pulsed) at 3 GHz
Efficiency : 5 – 20 % ( 30 % with depressed
collector)
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Applications of TWT
1.
2.
3.
4.
5.
Low noise RF amplifier in broad band microwave
receivers.
Repeater amplifier in wide band communication links
and long distance telephony.
Due to long tube life (50,000 hours against ¼th for
other types), TWT is power output tube in
communication satellite.
Continuous wave high power TWT’s are used in
troposcatter links (due to larger power and larger
bandwidths).
Used in Air borne and ship borne pulsed high power
radars.
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Biological effects of microwaves
 Electromagnetic radiation in the 1 mm to 1 m
wavelength range (300 MHz to 300 Ghz) is
referred to as microwave radiation.
 A part of which is known as radiofrequency (RF)
radiation, which covers 0.5 MHz to 300 GHz
range and is considered in the context of
adverse biological effects.
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Ionizing and non – ionizing radiations
of electromagnetic energy
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Ionizing radiation
 Ionization is a process by which electrons are
stripped from atoms and molecules and this can
produce molecular changes that can lead to
damage in biological tissue, including effects on
DNA, the genetic material.
 This process requires interaction with high levels
of electromagnetic energy to ionize biological
material, this include X-radiation and gamma
radiation.
 The energy levels associated with RF and
microwave radiations are not great enough to
cause the ionization of atoms and molecules,
therefore, it is a type of non-ionizing radiation.
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Non ionizing radiation
 Microwave energy is non-ionizing
electromagnetic radiation.
 Ionizing radiation messes up molecules, nonionizing radiation merely heats them.
 In general, it does not have sufficient energy to
kick an electron off an atom thus producing
charged particle in a body and cause biological
damage.
 The only proven harmful effect from exposure to
microwave (or RF) radiation is thermal.
 RF radiation can enter deep into the body and
heat human organs.
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Effect of microwaves in human body
 The blood vessels are dilating and the blood flow
increases substantially as the thermoregulatory
mechanism is activated in order to keep the body
temperature constant.
 With rising body temperature the metabolic rate rises,
which may lead to Stress-Adaptation-Fatigue
Syndrome.
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Effects produced by the electromagnetic
waves at different frequency level
 Above 10 GHz (3 cm wavelength or less) heating
occurs mainly in the outer skin surface.
 From 3 GHz to 10 GHz (10 cm to 3 cm) the
penetration is deeper and heating higher
 .From 150 MHz to about 1 GHz (200 cm to 25 cm
wavelength), penetration is even deeper and
because of high absorption, deep body heating
can occur.
 Any part of the body that cannot dissipate heat
efficiently or is heat sensitive may be damaged by
microwave radiation of sufficient power.
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Measurement of Microwave exposure
 The microwave energy exposure is measured in terms of
SAR (Specific Absorption Rate) or PD (Power Density).
 SAR is the energy which is absorbed in a unit of mass or
volume of the body per unit time.
 The standards that limit microwave exposure were set at
0.4 W/kg SAR for occupational and 0.08W/Kg for public
exposure.
 The averaging time for determination of SAR was 6
minutes. Power density is the energy absorbed per unit
area in unit time. The high power microwaves definitely
cause some adverse effects in the human system
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Effects of Microwave energy
Power
level
Long-term effect
(mW /cm2) on human body
Remarks
0.01
Nothing
0.1
Nothing
1
Nothing
5
Nothing
Accepted standard for microwave
oven leakage
10
Nothing
Accepted standard for maximum
continuous exposure to radiated
emissions (cell phones, etc.)
30
You can feel heat
100
Cataracts can be
produced
1000
Pain is induced
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Do you know YOUR Brain can be FRIED???
What do Microwave Ovens, Cell Phones and
Cordless Phones have in common?
They all emit... Dangerous Microwave
Radiation!
The GOOD NEWS is... with Microwave radiation
you can...
Boil water
Cook meat
Fry eggs
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The BAD NEWS is...
with Microwave radiation you can...
Fry Your Brain
Your head and brain heat up significantly when you talk
on your cell phone or cordless phone.
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Want proof?
After 15 minutes of using a cell phone, the orange, red and pink show
significant, dangerous HEAT. Most heat is generated in your ear canal,
which is directly connected to YOUR BRAIN
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After 15 minutes of using a cell phone WITH the BIOPRO
Harmonization Chip applied to it, the green and blue colors
show cool tissue.
Your head's temperature remains normal, providing you with the
protection you deserve.
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Proof enough?
Some scientists estimate that you are now exposed daily to
100 million times the electromagnetic frequency (Micro
wave) radiation of your grandparents.
So….
AVOID FREQUENT USE OF
CELL PHONES!!!
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